This invention pertains to high-emittance electron-beam sources for use in any of various electron-beam devices, particularly electron-beam microlithography apparatus and related exposure apparatus. Electron-beam microlithography is a type of charged-particle-beam microlithography, which represents one of several xe2x80x9cnext-generationxe2x80x9d microlithography technologies currently experiencing intensive development effort due to its potential for achieving substantially greater resolution than obtainable using conventional optical microlithography technology. Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, thin-film magnetic pickup heads, micromachines, and the like.
A conventional thermionic electron-beam source (also termed an xe2x80x9celectron gunxe2x80x9d) is shown in FIG. 7. The depicted source includes a cathode 1 (serving as the electron-emitting surface), a Wehnelt electrode 3, and an xe2x80x9cextraction electrodexe2x80x9d (anode) 4 arranged along an axis Ax. The cathode 1 normally is heated by a heating means (not shown but well understood in the art) to cause the cathode to emit hot electrons. The emitted electrons are formed into a beam 2 by the Wehnelt electrode 3 and anode 4. Specifically, the anode 4 extracts electrons emitted from the cathode 1 and urges them to propagate in a downstream direction (to the right in the figure) from the cathode to the anode 4 and beyond. Electrons of a beam 2 emitted from the cathode 1 and propagating initially parallel to the axis Ax (i.e., axially propagating electrons) are subjected to respective lens actions by the respective voltages applied to the Wehnelt electrode 3 and anode 4. The electrons of the beam 2 converge at a gun crossover 5. The axial position of the gun crossover 5 is a function of the respective voltages applied to the Wehnelt electrode 3 and anode 4 (e.g., a higher negative voltage applied to the Wehnelt electrode 3 will tend to move the gun crossover 5 to the left in the figure). In a microlithography context, the beam 2 propagating downstream of the anode 4 is acted upon by a downstream electron-optical system (not shown) that shapes and conditions the beam for use as an illumination beam for illuminating a desired region of a reticle or mask or other object (not shown).
In certain types of electron-beam apparatus the emittance of the electron gun is critical, especially if the apparatus is used for making one-shot lithographic exposures of respective portions of a pattern, or for making reduced (demagnified) transfer-exposures of a pattern. xe2x80x9cEmittancexe2x80x9d is a quantitative expression of the ability of the beam to achieve uniform irradiation of a defined surface, and is expressed as the range of uniform beam current in an area irradiated by the electron beam 2 multiplied by the aperture half-angle of the beam at the irradiated region.
In electron microscopes and microlithography apparatus utilizing an electron beam having a transversely Gaussian distribution but configured as a spot beam for pattern drawing, emittance normally is not a significant variable. This is because the area illuminated by a spot beam at any instant in time is only 1 to 10 nm in diameter, which is effectively at the apex of the distribution. In contrast, in microlithography apparatus utilizing a reticle divided into subfields that are exposed individually with demagnification, merely forming the beam to irradiate a spot is insufficient for achieving proper pattern transfer because the area illuminated by the electron beam is substantially larger than 1-10 nm across. Rather, it is necessary to achieve uniform irradiation in an area measuring 10 xcexcm square (typical of one-shot partial pattern block exposures) to 1 mm square (typical of one-shot reduced transfer exposures from respective subfields of a divided reticle). These latter areas encompass not only the apex of the Gaussian distribution but also the tails (distal or outlying portions) of the distribution. In addition, the aperture half-angle in these latter two cases is several mrad. As a result, to achieve the required uniform illumination over the desired one-shot area, high emittance from the electron gun is necessary.
To improve the transverse uniformity of the energy of the electron beam emitted from the electron gun used in apparatus for performing partial pattern block exposures and reduced transfer exposures from a divided reticle, the cathode normally is made transversely wide and planar as shown in FIG. 7. A wide planar cathode also improves the uniformity of beam current as incident on a substrate such as a semiconductor wafer when forming a microlithographic image of the cathode on the substrate. However, whenever electrons are emitted from a wide cathode surface, beam current tends to be excessive. Hence, various means conventionally are employed to prevent emission of extraneous electrons from the cathode. Exemplary conventional means include fabricating the cathode of a material having a high work function or applying a substance having a high work function to portions of the cathode surface located off axis.
If the cathode is a thermionic-emission type, the electron gun generally exhibits a relationship between emission-current density Jc and anode voltage Va as shown in FIG. 10. In the figure Tc is cathode temperature. For example, the relationship of Jc versus Va for Tc3 is indicated by the solid-line curve. The region where the relationship of Jc versus Va is nearly according to Jcxe2x88x9dVa2/3 is termed a xe2x80x9cspace-charge-limitedxe2x80x9d region. The more distal region is a xe2x80x9ctemperature-limitedxe2x80x9d region.
As the temperature of a thermionic cathode rises, beam current can become excessively high, and operation of the electron gun becomes space-charge limited. Whereas operation of the gun in a space-charge-limited manner can be performed in a stable manner, the presence of a high-charge field at or adjacent the cathode surface can cause the emitted electrons to lose characteristics reflective of the cathode surface from which they were emitted. If the electric field is substantially non-uniform, then electron emission from the cathode surface is not uniform. Under such conditions, the uniformity of current at the cathode surface conventionally cannot be utilized. Hence, there is a need for a way in which to utilize the electron gun in a temperature-limited region having a relatively low temperature and low beam current at the cathode.
Meanwhile, the distribution of the aperture angle is determined by the transverse energy distribution of the beam, which is determined by the cathode temperature of the cathode as electrons are being emitted from the cathode. The trajectories of electrons, emitted from the cathode 1 at a point of intersection of the optical axis Ax with the cathode surface, and emitted at an angle relative to the optical axis Ax are shown in FIG. 8. Near-axis trajectories 6 determine the configuration and dimensions of a crossover 7 formed at the crossover point 5. The spatial intensity of the beam at the crossover 7 is a function of the distribution of electron emission at the cathode surface, which (as discussed above) usually is a Gaussian distribution.
The emittance at a surface irradiated by the beam is determined not only by the uniformity of beam current and aperture angle at the cathode of the electron gun, but also by aberrations generated by lens actions generated by respective voltages applied to the electrodes of the electron gun. Whereas emittance can be preserved if the downstream optical system is free of aberrations, emittance is degraded by an optical system exhibiting significant aberrations. If emittance has deteriorated, it generally cannot be restored by the downstream optical system.
In conventional electron guns, substantial aberrations are imparted to the image at the crossover point 5 by the respective lens actions of the Wehnelt electrode 3 and the anode 4, as shown in FIG. 9, depicting a spherical aberration. Whenever spherical aberration is evident at the crossover point 5, the beam current is irregular and aperture angle becomes location-dependent at the wafer or other substrate. This, in turn, causes a substantial degradative deviation of actual emittance from the desired emittance, and results in a deterioration of the uniformity of imaging performed by the beam.
Therefore, there is a demand for electron-beam sources that exhibit substantially reduced spherical aberration at a crossover compared to conventional electron-beam sources.
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide electron-beam sources that exhibit substantially reduced spherical aberration compared to conventional sources.
To such end, and according to a first aspect of the invention, electron-beam sources are provided. An embodiment of an electron-beam source according to the invention comprises a cathode situated on an axis and configured to emit an electron beam propagating along the axis in a downstream direction from the cathode. The cathode comprises a transversely planar center portion and a transversely planar peripheral portion. The peripheral portion is xe2x80x9cdrawn backxe2x80x9d in an upstream direction along the axis relative to the center portion.
Drawing back the peripheral portion of the cathode relative to the center portion provides several advantages. One advantage is that peripheral portions of a beam exhibiting spherical aberrations can be effectively blocked using a downstream aperture situated at a crossover image plane. This results in a more uniform transverse beam-intensity distribution immediately downstream of this image plane. Another advantage is that a more uniform angular distribution of the beam at the reticle and wafer is obtained. Hence, if the beam source is used in an electron-beam microlithography apparatus, drawing back the peripheral portion of the cathode yields a corresponding improvement of the uniformity of beam current at the wafer and reduces the location-dependency of the aperture angle of the beam. This drawing back also allows the voltage applied to a Wehnelt electrode of the source to be reduced, yielding improved uniformity of the electrical field at the electron-emitting surface of the cathode, with a correspondingly improved uniformity of the distribution of electrons extracted from the electron-emitting surface. These benefits yield a substantially improved emittance compared to conventional electron-beam sources.
As suggested in the summary above, the subject electron-beam source can further include a Wehnelt electrode situated coaxially downstream of the cathode, and an extraction electrode (also termed an anode) situated coaxially downstream of the Wehnelt electrode.
According to another aspect of the invention, methods are provided (in the context of producing an electron beam using an electron gun including a cathode and a Wehnelt electrode arranged along an optical axis) for increasing uniformity of a field at an electron-emitting surface of the cathode. In an embodiment of such a method, the cathode is configured with a transversely planar center portion and a transversely planar peripheral portion. The peripheral portion is drawn back, relative to the center portion, in an upstream direction along the optical axis. This drawing back weakens a field imposed at the electron-emitting surface.
According to another aspect of the invention, electron-beam microlithography apparatus are provided that include an electron-beam source such as any of those, according to the invention, summarized above. Since the electron-beam source is characterized by having a high emittance, the subject apparatus is especially suitable for use in achieving uniformity of illumination of a selected region on a reticle. The subject apparatus also is especially suitable for use in lithographic situations requiring high emittance, such as partial pattern block exposure and/or reduced transfer-exposure of a pattern from a divided (xe2x80x9csegmentedxe2x80x9d) reticle.
An electron-beam microlithography apparatus according to the invention can include an illumination-optical system and a projection-optical system. The illumination-optical system is situated along the axis downstream of the source. The illumination-optical system is configured to receive the electron beam, propagating as an illumination beam from the source, and to direct the electron beam to a reticle so as to illuminate a region on the reticle. The projection-optical system is situated along the axis downstream of the reticle. The projection-optical system is configured to receive the electron beam, propagating as a patterned beam from the reticle, and to direct the patterned beam to a substrate (e.g., semiconductor xe2x80x9cwaferxe2x80x9d coated with a suitable resist).
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.